Extremely short duration optical pulses are important for high speed signal processing and communications. Optical pulses in this category are commonly called ultrashort optical pulses and have durations in the picosecond and sub-picosecond range. Ultrashort optical pulses have been obtained by techniques such as passive modelocking wherein a nonlinear element is placed either within the lasing cavity or within a cavity external to the lasing cavity. One such nonlinear element is a saturable absorber.
Saturable absorbers are materials which display a change in opacity related to the incident radiation intensity at a particular wavelength. Solids, liquids, and gasses may be used as saturable absorbers based upon the chosen wavelength of operation. The saturable absorber acts as a shutter. It absorbs all weak radiation incident upon it. As the intensity of the incident radiation reaches a sufficiently high level called the saturation intensity of the saturable absorber, the incident radiation is permitted to pass through the absorber. In general, the attenuation of the incident radiation caused by the saturable absorber is relatively low because the absorber is saturated into a transparent state at the wavelength for the incident radiation.
Organic materials such as dyes exhibit a broadband absorption response of several hundred nanometers. But, dyes require the use of mechanical elements such as nozzles and the like which are relatively large, subject to mechanical malfunction, and not easily integrable with solid state lasers. When used as saturable absorption elements, semiconductors form compact, reliable elements suitable for integration with solid state lasers. In contrast to organic materials, semiconductor materials are inherently narrowband saturable absorption elements having an absorption response of several tens of nanometers. Generally, semiconductor materials have a significantly narrower operating range than the tuning bandwidth of the laser system in which the semiconductor materials act as saturable absorbers. In order to modelock the laser for pulsed operation at a particular wavelength in the tuning range of a laser, it would be necessary to have a plurality of different replaceable narrowband saturable absorbers to cover the entire tuning range of wavelengths for the laser. Thus, there is no single solid state saturable absorber available having a sufficiently broadband absorption response which is substantially coextensive with the tuning range of available lasers.